September 8, 2011

On Wednesday, Sept. 7, the MESSENGER spacecraft emerged unscathed from the second of four “hot seasons” expected to occur during its one-year primary mission in orbit around Mercury. Hours later, mission controllers at the Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Md., successfully executed a maneuver to adjust the spacecraft´s trajectory.

“Operating MESSENGER in Mercury orbit is a bit like driving a high-performance automobile on a challenging track with continuously varying road conditions and unpredictable changes in weather,” says MESSENGER Principal Investigator Sean Solomon, of the Carnegie Institution of Washington. “Our road track is three-dimensional, our environment imposes extreme variations in temperature every 12 hours, and our weather comes from space. Nonetheless, our spacecraft is resilient, our operations team is experienced at handling all the needed maneuvers, and we´re continuing to learn how to squeeze as much science as possible from our orbital observations.”

This hot season began on August 9 and lasted nearly one month. During that time, the closest approach of the spacecraft to Mercury was on the sunlit side of the planet. MESSENGER´s sunshade reached temperatures as high as 350°C during this season, and its solar panels had to be turned off the Sun for short periods during each orbit to protect them from overheating.

Within this second hot season there was a period – from August 20 to August 29 – in which the spacecraft experienced an eclipse during the portion of each orbit when it passed through the shadow of the planet. For eclipses that last longer than 35 minutes, the probe´s battery cannot power the full science payload, and some instruments must be turned off for a brief period.

Even with the challenges of operating the satellite in these extreme orbital environments, the team was able to fine-tune the system to increase the scientific data return from Mercury, says MESSENGER Systems Engineer Eric Finnegan, of APL.

“We went into the first hot season with very conservative assumptions for the spacecraft´s response to the Mercury environment,” says Finnegan. “To conserve energy, only one of MESSENGER´s seven instruments was allowed to operate continuously throughout that season. As it turned out, the battery performed superbly, and there was sufficient energy margin to allow two additional instruments to maintain continuous measurements throughout the entire eclipse season.” In addition to the Gamma-Ray Spectrometer, the Neutron Spectrometer and the Magnetometer remained on continuously through the second hot season.

Having weathered the hot season, MESSENGER engineers immediately turned to the task of adjusting the spacecraft´s orbit around Mercury. This third of five expected orbit-correction maneuvers lowered the periapsis altitude from about 470 kilometers back to 200 kilometers.

“MESSENGER´s orbit is continuously changing, so correction maneuvers are scheduled throughout the year to ensure that the orbital parameters remain within the desired ranges for the planned science observations,” says MESSENGER Project Manager Peter Bedini. “This was the second time that the periapsis altitude – the closest approach of the spacecraft to the planet – has been reset to 200 kilometers. One more such correction is planned for the primary mission, and that event is presently scheduled to take place in early December.”

The 2-minute, 46-second engine burn began at 15:08 UTC (11:08 a.m. EDT) and reduced the period of the spacecraft's orbit around the innermost planet from 12 hours to 11 hours 46 minutes. The next orbit-correction maneuver, designed to restore the orbital period to 12 hours, is scheduled for October 24.